Most internal gear pumps used in vehicle transmissions by internal combustion engines and automatic motors use trochoidal teeth. With trochoidal teeth, the tooth surface of one of the outer and inner gears is arcuately defined while the tooth surface of the other gear is defined by non-slip rotation of the arcuately defined teeth of the one gear.
The internal gear pump according to the present invention uses a cycloidal tooth profile to discharge liquid or gas in an internal combustion engine or an automatic transmission. Such a pump is described in e.g. U.K. patent 233423 and German patent 3938346. The pump of the German patent is an internal gear pump having an outer gear (outer rotor) and an inner gear (inner rotor) having different numbers of teeth from each other. It takes advantage of excellent kinematics properties of teeth and tooth spaces having a perfect cycloidal tooth profile.
The teeth of the outer gear mesh with those of the inner gear driven by an engine crankshaft or the main shaft (spindle) of an automatic gear box. In this internal gear pump, relatively clear radial movement of e.g. the crankshaft as the drive shaft is compensated for by providing a suitable clearance between the periphery of the outer gear and the housing (i.e. providing a play that allows radial runout of the outer gear). For such compensation, the outer gear may be mounted with substantially no play but providing a correspondingly large play between the inner gear and a bearing of the inner gear. In this case, the inner gear teeth and the outer gear teeth are brought into mesh with each other. The concept of the present invention is suitably applicable to this type of pump.
FIG. 4 is a model view of a flattened cycloidal tooth profile proposed in unexamined Japanese patent publication 5-256268.
In the publication, in order to reduce noises resulting from pulsation of a discharge flow, drop in the pump efficiency, and cavitation noises, as observed in known pumps, the cycloidal tooth profile of each gear is flattened to reduce the gap between teeth at the portion where the outer and inner gears mesh most deeply with each other. In FIG. 4, fh represents an original epicycloid which is formed by the locus of a point on the circumference of a circle re when the circle rolls on a pitch circle P from the point z0, fr represents an original hypocycloid which is formed by the locus of a point on the circumference of a circle rh when the circle rolls on a pitch circle P from the point z0, while fh3 and rh3 represent an epicycloid and a hypocycloid after flattening, respectively.
Pressure pulsation of a hydraulic fluid, i.e. pulsation of discharge flow applies a vibrating force to the inner and outer gears, thus causing the teeth of these gears to collide against each other in radial and tangential directions, thus producing undesirable noises.
In unexamined Japanese patent publication 5-256268, trials are made to suppress such noises. But in the solution of this publication, the gap between teeth is extremely small at a portion where the outer and inner gears mesh most deeply with each other, and large at a portion where the depth of mesh between the gears is the shallowest. Thus, the gap is not uniform. This means that when pulsation of discharge flow occurs, the teeth of the gears tend to collide against each other at a portion where the depth of mesh between the outer and inner gears is the deepest. Noise suppression is thus not satisfactory.
Further, pointed tips (z1 and z2 in FIG. 4) are formed in the tooth profile. Such pointed tips tend to be chipped, increase the surface pressure represented by Hertzian stress, and promote wear of the tooth surface.
Discharge pulsation is not the only cause of these phenomena. In an ordinary internal gear pump, runout of the drive shaft coupled with the inner gear also causes noises and wear. Since the runout of the drive shaft is transmitted directly to the inner gear, this means that a vibrating force acts on the inner gear. Due to non-uniformity of the gaps between teeth, the teeth of the inner and outer gears tend to collide against each other.
Further, in the structure in which the gear teeth tend to collide against each other, a marked increase in the pulsation of discharge flow due to cavitation resulting from collapse of liquid or gas bubbles in the pumping chamber tends to promote such collision between gear teeth and thus increase noise and wear of tooth surface.
An object of the present invention is to provide an internal gear pump which can reduce noises and improve the mechanical efficiency and the life.